Highly pure monoalkylphosphine and method for producing same

ABSTRACT

A highly pure monoalkylphosphine which is useful as a starting material for producing a compound semiconductor, and a method for producing same in high yield are provided. A highly pure monoalkylphosphine represented by the general formula RPH 2  (wherein R is an alkyl group having 1 to 8 carbon atoms) has a purity of not less than five nines, and is substantially free of sulfur and silica. In the method of producing said highly pure monoalkylphosphine, anhydrous hydrofluoric acid is used as a catalyst for a reaction between phosphine and an alkene, and the reaction is carried out in the presence of an organic solvent having a boiling point higher than that of the resulting monoalkylphosphine; the resulting reaction mixture is contacted with an alkali solution so that the remaining catalyst is removed into an aqueous phase in the form of a fluoride salt; next, the obtained reaction mixture is contacted with an alkali hydride and the impurities are removed, then distillation is carried out.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a highly pure monoalkylphosphine whichis useful as a starting material for use in epitaxial growth by a MetalOrganic Chemical Vapor Deposition (MOCVD) method and the like, and amethod for producing same. More specifically, the present inventionrelates to a highly pure monoalkylphosphine which is substantially freeof sulfur or silica, and a method for producing same.

2. Description of the Related Art

Recently, compound semiconductors have been widely used in variousfields such as light emitting diodes, semiconductor lasers, and highelectron mobility transistors (HEMT). An epitaxial crystal-growthtechnique, such as a Metal Organic Chemical Vapor Deposition (MOCVD)method, has often been used as a method for preparing compoundsemiconductors. Compound semiconductors produced by such an epitaxialcrystal-growth technique include Group III-V compound semiconductors,and phosphine which contains phosphorus atom is used as a source ofGroup V elements.

However, phosphine has a safety problem since it is highly toxic and isin a gaseous form at ordinary temperatures. Accordingly, when phosphineis used for producing a compound semiconductor, it is necessary to useit under high pressure.

Recently, use of monoalkylphosphine in place of conventional phosphinehas been proposed in order to avoid the dangers associated withphosphine. Although it is not desirable for the epitaxial growth film tocontain carbon as an impurity, monoalkylphosphine allows only a smallamount of carbon to be mixed in the epitaxial growth film, and it has alower toxicity than phosphine; thus, monoalkylphosphine has beencatching attention as a substitute for phosphine.

As methods for preparing monoalkylphosphines, there have been known, forinstance, those comprising reducing phosphonium chloride and phosphonousacid, such as described in Z. anorg. allg. Chem. 433, 42 (1978) and thelike. Japanese Patent Laid-Open Nos.4-9392 and 4-9391 disclose methodsin which phosphonium chloride is produced by the Grignard method or theFriedel-Crafts method respectively, and is reduced by the use of areducing agent such as lithium aluminium hydride to give amonoalkylphosphine.

In these methods, it is very difficult to obtain a highly puremonoalkylphosphine since the metallic reducing agents used for thereduction reaction, a magnesium compound in the Grignard method and analuminium compound in the Friedel-Crafts method, tend to become includedas metal impurities. In addition, the production processes aremultistage processes which result in decreased yield and disadvantagesin the practice thereof on an industrial scale.

In the specification of U.S. Pat. No. 5,354,918, it is described that analkanesulfonic acid, such as methanesulfonic acid, is used as a catalystto produce phosphine and olefin. In this patent, the methanesulfonicacid catalyst used is mixed in an organic solvent containingmonoalkylphosphine. Thus a step to remove the catalyst, comprising, forexample, washing with an alkali aqueous solution such as sodiumhydroxide aqueous solution, becomes necessary. Nevertheless, even aftersuch procedures are carried out repeatedly, a trace amount ofmethanesulfonic acid remains and it is difficult to remove themethanesulfonic acid completely by subsequent purification processessuch as distillation and precision distillation.

If a trace amount of sulfur or silica is mixed in themonoalkylphosphine, the carrier concentration of the compoundsemiconductor formed from crystals prepared through the epitaxial growthof the monoalkylphosphine is lowered, and the product cannot be used forapplications requiring a high purity, such as an embedded laser.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a highlypure monoalkylphosphine which is useful as a starting material forproducing a compound semiconductor, and a method for producing thehighly pure monoalkylphosphine in high yield.

The present inventors have carried out an extensive study and solved theabove-mentioned conventional problems.

Accordingly, an object of the present invention is to provide a highlypure monoalkylphosphine which is represented by the general formula RPH₂(wherein R is an alkyl group having 1 to 8 carbon atoms) the purity ofwhich is not less than 99.999% (five nines), and which is substantiallyfree of sulfur and silica.

Another object of the present invention is to provide said highly puremonoalkylphosphine, wherein the monoalkylphosphine ismono-1,1-dimethylethylphosphine which is used as a starting material fora compound semiconductor.

Still another object of the present invention is to provide a method forproducing a highly pure monoalkylphosphine containing a step in whichphosphine and an alkene are allowed to react, which comprises:

step 1) in which anhydrous hydrofluoric acid is used as a reactioncatalyst and the reaction is carried out in the presence of an organicsolvent having a boiling point higher than that of the resultingmonoalkylphosphine;

step 2) in which the resulting reaction mixture is contacted with analkali solution and the remaining catalyst is removed in the aqueousphase as a fluoride salt; and

step 3) in which the reaction solution obtained in step 2 is contactedwith an alkali hydride to remove impurities, then distillation iscarried out.

A further object of the present invention is to provide said method forproducing a highly pure monoalkylphosphine, wherein the alkali solutionis an aqueous solution of a hydroxide of a Group Ia metal or Group IIametal, or an aqueous solution of ammonia or an amine type compound.

A still further object of the present invention is to provide saidmethod for producing a highly pure monoalkylphosphine, wherein the GroupIa metal is sodium or potassium.

A yet still further object of the present invention is to provide saidmethod for producing a highly pure monoalkylphosphine, wherein thealkali hydride is at least one substance selected from the groupconsisting of sodium hydride, aluminium lithium hydride and calciumhydride.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be further explained in detail.

The highly pure monoalkylphosphine of the present invention can berepresented by the general formula RPH₂. In the formula, R represents analkyl group having 1 to 8 carbon atoms, such as isopropyl group,tert-butyl group, tert-amyl group, and tert-octyl group, but thetert-butyl group is particularly preferable.

The monoalkylphosphine is highly pure, having a purity of not less thanfive nines, and substantially free of sulfur and silica.

The phrase "substantially free of-sulfur and silica," means that thecontent of sulfur and silica are below the detection limits whenmeasuring by an ICP-atomic emission spectroscopy method. "Below thedetection limit," means the values are not more than 10 ppbrespectively.

In one preferred embodiment of the present invention, the highly puremonoalkylphosphine of the present invention ismono-1,1-dimethylethylphosphine which is particularly preferable as astarting material for a compound semiconductor.

The method of the present invention will be more concretely explained.

Phosphine, which is the starting material used in the first step of thepresent invention, can be produced by any production method, but it ispreferable to use a product of a high purity which contains very littlemetallic and oxidative impurities. Industrially advantageous andpreferable is, for example, purified phosphine which is obtained fromcrude phosphine by-produced in a soda hypophosphite production process,subjected to an arsine removal process or lower phosphorus hydridecompound removal process, followed by precision distillation in adistilling apparatus so that it becomes substantially free of impuritiessuch as low boiling point components, CO₂, H₂ O, and arsine.

Alkenes, which are used as another starting material, are unsaturatedaliphatic hydrocarbons having a straight or branched chain, and thosehaving 1 to 8 carbon atoms are preferable. Examples thereof includeisobutene, 2-methyl-1-butene, 2-ethyl-1-butene, 2-methyl-1-pentene,2-methyl-1-hexene, 2,3,3-trimethyl-1-butene, 2,3-dimethyl-1-hexene,2-ethyl-1-hexene, isooctene, 2-methyl-1-heptene,2,2,4-trimethyl-1-pentene, 2,4-dimethyl-1-hexene,2,4,4-trimethyl-1-hexene and the like.

One of the features of step 1 of the present invention is that anhydroushydrofluoric acid is used as the catalyst in the reaction between theabove-mentioned compounds.

Another important feature of step 1 is that a solvent having a boilingpoint higher than that of the resulting monoalkylphosphine is employedas the reaction solvent. This is because the difference in boilingpoints is utilized to remove the highly pure monoalkylphosphine from thereaction solvent by distillation or, if necessary, precisiondistillation, which is carried out after the completion of the reaction.As the reaction solvent, saturated aliphatic hydrocarbons are suitable,and in particular, saturated aliphatic hydrocarbons having 8 to 18carbon atoms are preferable. Examples include n-octane, isooctane,n-nonane, n-decane, n-tridecane, n-tetradecane, n-hexadecane,n-octadecane and the like. Mixed solvents such as n-paraffin can be usedas well.

The other feature of step I of the present invention is that theabove-mentioned reaction is carried out in a water-free system. Bycarrying out the reaction in a water-free system, the generation ofsecondary or tertiary alkylphosphine compounds, which are inevitablyby-produced in an aqueous system, can be controlled.

The reaction conditions vary depending on the physical properties of thestarting materials, the solvent, and the catalyst selected, but as anexample, reaction is carried out under pressure (for example a gaugepressure of 10-30 kg/cm³) in a high pressure vessel such as anautoclave, an alkene/phosphine molar ratio of from 1:1 to 1:5,preferably 1:1 to 1:2.5. The amount of the catalyst to be added can bedecided appropriately, and in an illustrative example for an alkene, itis from 0.1 to 2.0 moles, preferably 0.5 to 1.5 moles. The reactiontemperature is between room temperature and 100° C., preferably between20° and 40° C., and the reaction time is normally from 1 to 24 hours,preferably 2 to 10 hours.

The introduction of the starting materials can be favorably carried outby introducing a reaction solvent into a reaction vessel such as anautoclave, sufficiently replacing the atmosphere in the reaction vesselwith an inactive gas such as nitrogen and helium, and introducing areaction catalyst then phosphine into the reaction vessel under pressurewhile the reaction vessel temperature is lowered to as low as about -1°C. Further, it is desirable that an alkene dissolved in the reactionsolvent or an alkene alone be introduced into another autoclave,pressurized using an inactive gas such as nitrogen, then graduallyintroduced under pressure into the reaction vessel containing thephosphine.

After the reaction is completed and the reaction vessel is cooled downto room temperature, the excess amount of unreacted phosphine isreplaced with an inactive gas followed by allowing the reaction solutionto stand for about 24 hours.

Next, step 2 is a step in which unreacted anhydrous hydrofluoric acidremaining in the reaction solution is removed by being brought intocontact with an alkali solution.

Examples of the alkali solution to be used include an aqueous solutionof a hydroxide of Group Ia metals, Group IIa metals, and ammonia oramine type compounds.

The concentration of the aqueous solution is preferably from 0.1 to 3N.The amine type compounds hereby include aliphatic amine compoundssubstituted with an alkyl group having up to 4 carbon atoms, urea andtetramethyl urea, i.e. a derivative of urea. These alkali solutions canbe used alone or in combination. Among them, aqueous solutions ofhydroxides of Group Ia metals, preferably sodium and potassium, havingconcentrations ranging from 0.5 to 2N are particularly preferable. Forpreparation of the aqueous solution, an alkali compound of high purityis preferably dissolved in extra-pure water.

After addition of the alkali solution, the reaction solution is stirredat a normal temperature or under heating. The amount of the alkalisolution added is 1.0-3.0 moles, preferably 1.1-1.5 moles per 1 mole ofhydrofluoric acid used as the catalyst. By such a process, the anhydroushydrofluoric acid used as the catalyst can be sufficiently removed.

Next, step 3 is a step in which the reaction solution treated in theprevious step is brought into contact with an alkali hydride to removeimpurities, and distillation is carried out.

Examples of the alkali hydride to be used include sodium hydride,aluminium lithium hydride or calcium hydride.

The contact is preferably carried out by adding an alkali hydride to thereaction solution followed by reflux treatment in an inactive gasatmosphere such as nitrogen or argon. By this treatment, impurities suchas water and alcohol can be removed.

Then the reaction solution (organic phase) after the treatment issubjected to simple distillation under a normal pressure and, ifnecessary, further purified by precision distillation to give a highlypure monoalkylphosphine which is free of impurities.

The addition of an alkali solution, the stirring, the treatment with thealkali hydride, and the distillation steps can be repeatedly carriedout.

Since neither sulfur compounds nor silicon compounds are used in thereaction according to the present invention, a monoalkylphosphine ofextremely high purity can be obtained, because sulfur and silica, whichare said to degrade the quality of crystal growth, are not contained inthe monoalkylphosphine.

The present invention improves the conventional method of producing amonoalkylphosphine wherein phosphine and an alkene are allowed to react,and the method of the present invention allows a highly selective andhighly pure monoalkylphosphine which is free from sulfur and silicaimpurities to be advantagously produced industrially. Themonoalkylphosphine obtained according to the present invention can beeffectively utilized as a source of phosphorus, a Group V element for acompound semiconductor.

EXAMPLES

The present invention will hereinafter be described in more detail inthe following Examples.

Example 1

Into a stainless autoclave having a capacity of about 10 liters (firstautoclave), was added 2 liters of n-paraffin SL (produced by NipponPetrochemical Co., Ltd.) as a solvent. Then, the vessel was purged bythoroughly replacing the atmosphere therein with nitrogen and then avacuum. Next, a bomb of anhydrous hydrofluoric acid was prepared in ahot bath of 35° C. and the first autoclave, while being cooled to -10°C. with cold brine, was charged with 200 g of the anhydrous hydrofluoricacid (10 moles) from the bomb. Then, the first autoclave, while beingcooled with cold brine, was further charged with 952 g (28 moles) ofpurified phosphine gas which was by-produced in a soda phosphiteproduction process. The pressure level as shown by the pressure gage ofthe first autoclave, rose to 22 kg/cm². Next, the first autoclave washeated with hot water to 40° C .

A separate autoclave having a capacity of about 5 liters (secondautoclave) was charged with 2 liters of a solvent, n-paraffin SL(produced by Nippon Petrochemical Co., Ltd.) and 560 g (10 moles) ofisobutylene. Then, after they were dissolved, the inner pressure of thesecond autoclave was raised to 30 kg/cm² using nitrogen. Next,isobutylene solution was gradually introduced into the first autoclaveunder pressure over 1 hour by adjusting the valve. The pressure in thefirst autoclave was reduced to 27-18 kg/cm2. Then the reaction mixturein the first autoclave was matured at 40° C. for 3 hours.

A neutralization bath having a capacity of 20 liters and a stainlessagitator was charged with 12 liters of 1N sodium hydroxide aqueoussolution, then cooled to 10° C. or less using an ice bath. The reactionmixture in the first autoclave was gradually added to the neutralizationbath under stirring, and the hydrofluoric acid, used as the catalyst,was neutralized. After the reaction product was withdrawn, it wasallowed to stand at room temperature over an entire day. Next, aseparated lower phase comprising a sodium fluoride aqueous solution wasremoved, and the organic phase was further washed with 10 liters of 1Nsodium hydroxide aqueous solution.

The obtained organic phase was analyzed by gas chromatography and 382.9g of mono-1,1-dimethylethylphosphine was obtained (conversion rate:42.5% on the basis of isobutylene).

To the organic phase, was added 20 g of calcium hydride and it wasrefluxed under a nitrogen atmosphere for 3 hours, then distilled at anormal pressure to give 350.0 g of a distillate having amono-1,1-dimethylethylphosphine content of 93%. This was subjected toprecision distillation at a normal pressure using a 15-stage Aldershowglass precision distillatory, and 245.9 g (yield: 27.3%) ofmono-1,1-dimethylethylphosphine (boiling point: 54° C.) was isolatedwhich had a purity of at least 99.999% according to the analyticalresults of gas chromatography.

An InP epitaxial film (Fe dope, film thickness of 5 μm) was grown undera normal pressure on an InP substrate from the obtainedmono-1,1-dimethylethylphosphine and from commercially available highlypure trimethyl indium using a horizontal MOCVD apparatus. The crystalgrowth was carried out at 580° C., and at V/III ratio of 50. Theelectrical characteristics of the obtained epitaxial film were measuredand;

the carrier concentration was n77 k=2-3×10¹⁴ cm⁻³ ; the surface homologywas also very good.

The purity of the product obtained by relative area ratio as measured bygas chromatography (FID detector) was 99.999% or higher, and the amountsof sulfur and silica measured by ICP-atomic emission spectroscopy werebelow the detection limit (not more than 10 ppb).

Comparative Example 1

Into a stainless autoclave having a capacity of about 10 liters, wereadded 100 g of n-decane (boiling point: 174° C.), 80 g of isobutylene(1.426 mole) and 135.8 g (3.994 moles) of highly pure phosphine at roomtemperature. The pressure of the autoclave was 25 kg/cm². The reactiontemperature was raised to 60° C. and 137 g (1.426 mole) ofmethanesulfonic acid purified by simple distillation was added theretoover about 1 hour by pressure pump. The pressure inside the autoclavewas lowered from 35 kg/cm² down to 28 kg/cm². Again, the reactionmixture was kept at 60° C. for 1 hour and matured.

After the reaction was completed, the reaction mixture was cooled toabout 30° C., the excess unreacted phosphine was removed throughevacuation, and the atmosphere of the system was thoroughly replacedwith nitrogen. The reaction product was allowed to stand at roomtemperature for an entire day, and then liquid-liquid separation wascarried out and the lower phase comprising methanesulfonic acid wasremoved.

Next, 150 g of 1N sodium hydroxide aqueous solution was added theretoand stirred for 1 hour, allowed to stand, and liquid-liquid separationwas carried out.

The obtained n-decane phase was analyzed by gas chromatography and itwas found that 56.1 g of mono-1,1-dimethylethylphosphine (conversionrate: 43.7% on the basis of isobutylene) was obtained.

To this n-decane phase, was added 5 g of calcium hydride and the mixturewas refluxed under a nitrogen atmosphere for 3 hours in a manneranalogous to that used in Example 1; then distillation at a normalpressure was carried out, followed by precision distillation at a normalpressure, and 33.6 g (yield: 26.2%) of mono-1,1-dimethylethylphosphine(boiling point: 54° C.) having a purity of 99.99% or higher, measured bygas chromatography, was isolated.

InP epitaxial film was grown under a normal pressure on an InP substrateusing a MOCVD apparatus similar to that used in Example 1, and theelectric characteristics were measured and;

the carrier concentration was n77 k=0.8-1.0×10¹⁵ cm⁻³.

The purity of the product obtained by relative area ratio measured bygas chromatography (FID detector) was 99.99%, and the amounts of sulfurand silica measured by ICP-atomic emission spectroscopy were, sulfur:150 ppb, and silica: 80 ppb.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one of ordinaryskill in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. A method for producing a monoalkylphosphinecontaining a step in which phosphine and an alkene are allowed to react,which comprises:step 1) in which anhydrous hydrofluoric acid is used asa reaction catalyst and the reaction is carried out in the presence ofan organic solvent having a boiling point higher than that of resultingmonoalkylphosphine; step 2) in which the resulting reaction mixture iscontacted with an alkali solution and the remaining catalyst is removedin the aqueous phase as a fluoride salt; and step 3) in which thereaction solution obtained in the step 2) is contacted with an alkalihydride to remove impurities, and then distillation is carried out.
 2. Amethod for producing a highly pure monoalkylphosphine according to claim1, wherein the alkali solution is an aqueous solution of a hydroxide ofa Group Ia metal or Group IIa metal, or an aqueous solution of ammoniaor an amine type compound.
 3. A method for producing a highly puremonoalkylphosphine according to claim 2, wherein the Group Ia metal issodium or potassium.
 4. A method for producing a highly puremonoalkylphosphine according to claim 1, wherein the alkali hydride isat least one substance selected from the group consisting of sodiumhydride, aluminium lithium hydride and calcium hydride.